Modelling of ignition of liquid hydrocarbon droplets at high pressure.

Abstract

This work presents a mathematical model for the auto-ignition process of a single droplet of pure liquid hydrocarbon at high pressure. The model solves the full transient equations of continuity, species diffusion and energy in the vapour phase surrounding the droplet using finite difference techniques. The Peng-Robinson equation of state is used to describe vapour-liquid equilibrium at the droplet surface. The effects on droplet evaporation and ignition of pressure, temperature, and liquid phase diffusion of dissolved air are discussed. The model was tested with a droplet size of 1.5 mm and an ambient temperature of 973 K and pressure varying from 1 to 50 atm. For a pure (single component) fuels the ignition delay time was found to be a strong function of pressure, temperature and diameter. The ignition time decreases as pressure rises. At high pressure the reaction zone moves closer to the droplet surface. The air diffusion in the liquid phase and correction of transport properties for the effects of pressure have only small effects on ignition time. The model shows that the droplet cannot reach or pass the critical state but can only approach it asymptotically. The ignition behaviour of a two-component mixture is strongly controlled by the more volatile component. A change in pressure does not have significant effects on the behaviour of ignition time with mixture composition. Liquid diffusion has no large effect on the ignition time, but may affect the subsequent combustion processes.